In general, a liquid crystal display device has two substrates each having transparent electrodes and a liquid crystal sandwiched between the substrates. The liquid crystal display device is enabled for display in a desired manner by applying a predetermined voltage between the transparent electrodes to drive the liquid crystal and to thereby control light transmittance at each pixel. Recently, there is increasing demand for liquid crystal display devices, which has diversified requirements for liquid crystal display devices. Above all, improvement of display quality is strongly demanded.
Presently, active matrix type liquid crystal display devices having a thin film transistor (TFT) at each pixel as a switching element (TFT-LCDs) have become the main stream of the field. In a TFT-LCD, an interval between the two substrates (cell thickness) is maintained by spherical spacers or bar-shaped spacers made of plastic or glass. Such spacers are normally dispersed on either of the substrates at a spacer dispersing step before the substrates are combined. Thereafter, the two substrates are combined and are pressed from the outside such that the cell thickness is maintained at a value similar to the diameter of the spacers.
However, spacers dispersed within a pixel can cause an alignment defect of the liquid crystal and leakage of light. An alignment defect or leakage of light results in a reduction of contrast or glare on the display screen, which degrades display quality. Further, increases in the size of substrates have made it difficult to disperse spacers evenly. When spacers are unevenly dispersed, the cell thickness between the substrates can vary to result in irregularities of luminance. Particularly, in the case of such as an IPS (In-Plane Switching) or MVA (Multi-domain Vertical Alignment) mode liquid crystal display device, variation of luminance in response to variation of the cell thickness is more significant than that in a TN (Twisted Nematic) mode liquid crystal display device. Therefore, especially in the IPS or MVA mode liquid crystal display device, the cell thickness must be controlled to achieve a higher degree of uniformity in order to provide display without luminance irregularities. Further, since the trend toward pixels of higher definition has resulted in a reduction of the area of each pixel, the area occupied by spacers relative to that of pixels has increased, and spacers have now more significant influence on display quality
The recent trend toward substrates in greater sizes and pixels of high definition has resulted in the use of pillar spacers (resin spacers) made of a photosensitive resin instead of spherical spacers and bar-shaped spacers. Since pillar spacers are formed at a photolithographic step, they can be disposed in a region that is shielded from light with a light-shielding film (a black matrix (BM)) in an arbitrary disposing density. Therefore, neither alignment defect of the liquid crystal nor leakage of light occurs in pixels, there is no reduction in contrast or occurrence of glare. Further, since pillar spacers can be formed with a uniform thickness (height) compared with irregularity of a particle size in the spherical spacer or the like, control can be performed to obtain a uniform and accurate cell thickness between substrates. Therefore, no luminance irregularity attributable to variation of the cell thickness occurs. As described above, a liquid crystal display device utilizing pillar spacers can achieve display quality higher than those of a liquid crystal display device utilizing spherical spacers or bar-shaped spacers.
The pillar spacer is usually formed on a portion shielded from light by a BM. FIG. 10 is a process sectional view illustrating a manufacturing method of a known color filter (CF) substrate using the pillar spacer. First, as shown in FIG. 10(a), a BM 145 is formed of Cr metal or black resin on the insulating substrate 111 such as glass. Next, as shown in FIG. 10(b), CF resin layers 140R, 140G, and 140B having the colors of red (R), green (G), blue (B), respectively, is sequentially formed by using a pigment-dispersion type colored photosensitive resin and the like. Next, as shown in FIG. 10(c), a common electrode 141 is formed by sputtering a transparent electrode such as ITO. Next, as shown in FIG. 10(d), for example, a negative type photo resist based on acrylic resin is coated on the substrate, and a pillar spacer 146 having a predetermined size is formed on a predetermined position so as to have a predetermined distribution density by using a photolithographic method. Alignment films 151 and 150 are formed on a CF substrate 104 and a TFT substrate 102 in which a pixel electrode 116 is formed for every pixel, respectively, and the layers are processed by rubbing. Then, the both substrates 104 and 102 are assembled to each other and formed vacant cells. Next, a liquid crystal 106 is injected in the vacant cells, so that a liquid crystal display panel as shown in FIG. 11 is manufactured. As shown in FIG. 11, a cell gap between the substrates 104 and 102 is sustained by a structure 148 mainly including the pillar spacer 146 and the CF resin layer 140.
Here, in designing the distribution density of the pillar spacer 146, physical properties such as pressure displacement of a material or plastic deformation amount are important. It is necessary for the pillar spacer 146 to design so as to have both of a soft property for following heat expansion and contraction of the liquid crystal and a hard property for obtaining pressure resistance. For this reason, the pillar spacer 146 is disposed so as to have the distribution density which is usually one per several pixels as shown in FIG. 12.
Specifically, since “the pressure displacement amount per unit area of the pillar spacer” is uniformly designed, it is necessary for this to set to an optimum value on the basis of a cell process design. That is, cell hardness depends on “the pressure displacement per unit area of the pillar spacer” after cell combination. It is necessary for the pillar spacer to be compressed so as to have both of a soft property for following heat expansion and contraction of the liquid crystal and a hard property for undergoing pressure.
When the pillar spacer 146 is too hard, vacuum regions can be formed because the pillar spacer 146 cannot follow a decrease in the volume of the liquid crystal attributable to thermal shrinkage at a low temperature, and bubbles can be generated in such regions. At a liquid crystal filling and sealing step utilizing a dip type vacuum filling process, a panel which has been filled with a liquid crystal is pressed from the outside at a predetermined pressure to eject any extra part of the liquid crystal, whereby the cell thickness is adjusted However, when the pillar spacer 146 is too hard, the pillar spacer 146 cannot be sufficiently shrunk even if they are pressed at the predetermined pressure. Therefore, when the volume of the liquid crystal increases as a result of thermal expansion at a high temperature, the pillar spacer 146 cannot follow the increase in the volume of the liquid crystal. As a result, the liquid crystal is moved toward a lower part of the panel by gravity, and an irregularity is therefore caused by gravity in that the cell thickness becomes greater in the lower part.
When the pillar spacer 146 is too soft, since they undergo a great amount of displacement and also a great amount of plastic deformation in response to a pressure from the outside, an irregularity of the cell thickness can occur.
Recently, as for the method for realizing a decrease in time of the liquid crystal filling, one drop filling (ODF) method for performing simultaneously the substrate assembly and the liquid crystal filling has been used. In the one drop filling method, the cell thickness is determined by a volume of the liquid crystal. For this reason, it is necessary to match height of the pillar spacer 146 to the cell thickness determined by the volume of the liquid crystal. Considering manufacturing precision, it is also necessary to design the pillar spacer 146 so as to make the spacer even more softly.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-333628
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-148633